Stabilization of thin-shell implosions using a high-foot adiabat-shaped drive at the National Ignition Facility

Abstract

The thin-shell adiabat-shaped implosions proposed in this paper are designed to combine the ablation front stability benefits of the High Foot (HF) pulses with the demonstrated high fuel compressibility of the low foot implosions to reach the alpha-heating regime. This is accomplished by both lowering the drive between the first and second shocks and tailoring the rise-to-peak drive. Two-dimensional radiation hydrodynamics simulations show that while weakening the growth of low-mode number perturbations at the ablation front, this approach also introduces negative lobes to the growth factor spectrum at high mode numbers. A very-high foot picketless drive, characterized by an intermediate fuel adiabat level, is proposed to suppress negative perturbation growth. Moreover, the picketless feature of this design and the shorter duration of the through reduce the hohlraum wall motion allowing us to keep the capsule implosion symmetry under control. Introducing an accurately tuned dopant fraction in the outer ablator suggests that the stabilization of the ablation front may be even further improved. This study has shown that the smaller oscillation amplitude and the frequency of ablative Richtmyer-Meshkov instability reduce the initial perturbation seed at the beginning of the acceleration phase. The combination of a thin-shell design and a very high-foot picketless radiation drive has enlightened the calculated benefits of this intermediate fuel adiabat design: high implosion performance, more predictive low-mode implosion symmetry, and a similar stability at the ablation front than that of HF designs.